Composite infusions, such as, for example, VARTM, are closed-mold processes for fabricating large fiber-reinforced composite structures. In its simplest manifestation of composite infusions, a laminate fiber preform is installed onto a mold surface and sealed with an outer mold surface, for example, an outer sheet of flexible bagging material such as nylon or Mylar plastic. In VARTM, a vacuum is applied to remove entrapped air from the preform and resin is then allowed to infuse into the preform and cure. As the typical thermoset resins utilized for composite fabrication tend to have high viscosities (generally 150 centipoise (cp) or greater), processing techniques have been developed to improve the speed and quality of resin infusion. In particular, a variety of types of resin distribution media have been developed to promote resin flow.
There are three basic types of VARTM. Type 1 utilizes a resin distribution media over the top of the laminate, between the preform and the bagging material. Type 2 uses a sandwich core as a resin distribution media within the laminate. Type 3 uses specialized materials within the laminate itself as resin distribution media which unlike Type 1 stay in the composite component. For example, in Type 1 VARTM, the material is carried over the laminate in the x-y plane in the resin distribution media (a very permeable layer) and allowed to percolate or flow down into the laminate in the z direction through an easily separated layer (peel ply) to completely fill the laminate with resin. This minimizes the actual through-ply flow required for the thickness or z direction. Typical infusion resins have high viscosities (typically 200-600 cp at 25° C.), so choice of the correct resin distribution media for over-the-top flow is required to strike a balance between flow in the x-y plane and through-ply flow in the z direction.
As described in, for example, U.S. Pat. Nos. 5,840,238, 6,310,121, and 6,525,125, the disclosures of each of which are incorporated herein by reference, polymers generated by olefin metathesis processes are attractive as composite matrix materials. Of particularly beneficial use are the polymers generated by the ring opening metathesis polymerization (ROMP) of cyclic olefins. The low viscosity of cyclic olefin resin formulations and the ability to control ROMP kinetics (e.g., U.S. Pat. Nos. 4,708,969 and 5,939,504, the disclosures of which are incorporated herein by reference) facilitate composite processing and manufacture, and the corrosion resistance and high toughness of ROMP polymers leads to good composite durability. Commercially important ROMP resin formulations are generally based on readily available and inexpensive cyclic olefins such as dicyclopentadiene (DCPD), norbornenes, cyclooctadiene (COD), and various cycloalkenes.
Although the extremely low viscosities of ROMP resin formulations are attractive for rapid VARTM processing, they also present unique challenges. For example, typical high-viscosity resins tend to be slow-paced and self-correcting and forgiving. Voids and channels fill slowly and problems with competing flow rates due to differences in permeability within parts of the laminate are minimized. However, when one changes to ROMP resins with 1/10th to 1/20th or less of this viscosity, flow control issues are magnified and as a result most of the techniques used with the more viscous resins no longer yield acceptable results. FIG. 1(a) shows a simple depiction of an infusion set-up containing a resin distribution media (1), a reinforcement layer (2), and a mold surface (3). As resin is introduced into this evacuated infusion set-up in FIG. 1(b), resin flows rapidly along the resin distribution media (1) (x-y plane) and infuses more slowly into the reinforcement layer (2) (z direction) due to the permeability difference between the resin distribution media layer (1) and the reinforcement layer (2). This permeability difference may create a severe lead-lag (4), leading to areas within the reinforcement layer (2) with poor resin impregnation and possible void formation. As shown in FIGS. 1(c) and 1(d), as the resin continues to flow along the resin distribution media (1) (x-y plane) and infuses into the reinforcement layer (2) (z direction), the lead-lag may lead to areas of poor resin impregnation (i.e., dry spots or voids) (5). These areas of poor resin impregnation can lead to poor results, reduced mechanical properties, rejected parts, etc.
One of the tenets of resin infusion methods, such as VARTM, or any process involving liquid movement through a permeable media, is that liquid will follow the path of least resistance. Further, once such a path is established, backfill of unfilled areas is usually impossible. In composites, an unfilled part is a failed part. Whereas thicker, higher viscosity resins will be self correcting in this regard, lower viscosity resins (typically less than 100 cp at 40° C., for example, 1-50 cp, 5-25 cp, or 10-20 cp at 40° C.) require greater control. The current invention describes the incorporation of lower-permeability resin flow control structures to moderate the flow of resin (e.g., flow rates, flow direction, etc.) through resin distribution media layers and ensure full “wet out” (i.e., infusion of the desired amount of resin into the laminate to achieve the desired fiber volume in the composite) of all lamina. However, these pause points must be a balance of delay and promotion of flow to allow a full infusion of all lamina. Whereas the majority of VARTM improvements are aimed at trying to promote flow (i.e., increase the infusion rate because of the high viscosity of the resins), the low-viscosity resins require a balance of resin flow rates to allow optimal composite fill time while maintaining full and complete resin infusion into the reinforcement layers. Control of the flow in this manner ensures full and complete infusion without dry areas. One should remember that until the current generation of low-viscosity resins (e.g., ROMP resins), such resin flow techniques were unnecessary.
The invention describes the incorporation of resin flow control structures in an RTM process, such as VARTM infusion, allowing improved control of resin flow patterns with low-viscosity resins. Use of resin distribution media with high resin permeability allows for rapid resin delivery to key areas of the composite. Addition of resin flow control structures allows for modification of the resin flow in the distribution media, allowing control over resin lead-lag and resin channeling patterns to ensure full resin impregnation of the composite and to prevent voids and dry spots. This control is a combination of materials, process, and technique.